1. Technical Field
This invention relates to a multilayer ceramic substrate and its production method. To be more specific, this invention relates to an improvement in a multilayer ceramic substrate having a cavity formed therein.
2. Background Art
FIG. 4 is a cross sectional view of a conventional multilayer ceramic substrate 1. In FIG. 4, the multilayer ceramic substrate 1 comprises a laminate of ceramic layers 1a, 1b, 1c, 1d, 1e, 1f, 1g, and 1h disposed one on another. This laminate has end surfaces on opposite ends in the thickness direction of the laminate, and the laminate is formed with a cavity 2 which opens to at least one end surface. The cavity 2 is adapted for accommodating a chip device (not shown) such as a semiconductor IC chip.
The laminate has wiring conductors at predetermined positions of the ceramic layers 1a to 1h. Exemplary such wiring conductors include an external conductor films 6 formed on the end surface of the laminate, an internal conductor films 3 formed along the some of the interfaces between the ceramic layers 1a to 1h, a via hole conductor (through hole) 4 formed to penetrate through some of the ceramic layer 1a to 1h. 
The cavity 2 is formed by continuation of through opening formed in some consecutive ceramic layers of the ceramic layers 1a to 1h. 
In producing the multilayer ceramic substrate 1 shown in FIG. 4, ceramic green sheets 1a, 1b, 1c, 1d, 1e, 1f, 1g, and 1h corresponding to the ceramic layer 1a to 1h are first produced, and then, the wiring conductors as described above, namely, the external conductor films 6, the internal conductor films 3, and the via hole conductors 4 are formed on the predetermined sheets of the ceramic green sheets 1a to 1h at the predetermined position. The through opening for the cavity 2 is also formed in the predetermined consecutive ceramic layers of the ceramic layers 1a to 1h. 
After forming the through opening in the required ceramic green sheets, the ceramic green sheets 1a to 1h are overlaid one on another so that the opening of the cavity 2 formed by the continuation of the through openings is located at one end surface located at one end in the thickness direction of the laminate. A green sheet laminate is thereby produced.
The green sheet laminate 1 is then pressed in the thickness direction. The pressed green sheet laminate 1 is then fired to obtain the multilayer ceramic substrate 1 shown in FIG. 4.
However, when the green sheet laminate is simply pressed in thickness direction, inner periphery of the cavity 2 is likely to become swollen or collapsed due to the softness of the ceramic green sheets 1a to 1e. In addition, the bottom of the cavity 2 will escape the pressing, and the overall sheet laminate will not be pressed uniformly. In addition, the ceramic green sheets 1a to 1e is likely to become dislocated by slippage, and it is the top ceramic green sheet 1a that is most likely to become dislocated.
In order prevent such deformation of the cavity, the mold used for pressing may be provided with a projection which has a height corresponding to the depth of the cavity formed in the green sheet laminate, and the interior of the cavity may be compressed with this projection. However, overall compression ratio of the laminate will not be uniform even if the projection having a height identical with the depth of the cavity in the green sheet laminate were provided, since the stroke in the compression of the part of the laminate other than the part corresponding to the projection is large and this results in the increased compression ratio of the part of the green sheet laminate corresponding to the projection.
It is also quite difficult to provide the projection with a mechanism that would adjust the stroke or the compression ratio since such mechanism inevitably invites complication of the mold structure. Such complicated mechanism may be provided if only one cavity were to be formed. The mold used in mass production, however, is usually provided with a large number (typically about 400 to 600) of projections since many substrates are pressed at once, and provision of such complicated mechanism to all of the projections is not at all realistic in terms of the production convenience and the cost if such provision were not at all impossible.
Japanese Patent Application Laid-Open No. (JP-A) 2001-267448 and other documents recite isostatic pressing using an intervening elastic member to thereby apply a consistent pressure in the compression. It is, however, difficult to form corners of the cavity or corners of the substrate by such isostatic pressing using the intervening elastic member, and the deformation is likely to take place. Surface irregularity and waving is also likely to be found in the bottom of the cavity, and flip chip packaging of the device is difficult in such a case.
JP-A 2001-230548 discloses use of the so called “non-shrinking ceramic sheet”. Use of such material, however, only prevents shrinkage of the substrate in lengthwise direction, and shrinkage in the thickness direction is not regulated. In this case, the L and/or C elements formed by the internal conductors are less likely to enjoy improved precision.